Chemical mixer

ABSTRACT

Apparatus for mixing dry chemical with a liquid including a mixing tank having a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank. Wetting means is provided for wetting the dry chemical as it falls into the mixing tank, the wetting means including first spray means located on one side of the path of the dry chemical and second spray means located on the opposite side of the path of the dry chemical. The streams of liquid from the first and second spray means intercept each other substantially in the path of the dry chemical so that the dry chemical passes through the intercepting streams and is wetted thereby as it falls into the mixing tank. The apparatus further includes a detention tank located beneath the mixing tank, and valve means for controlling communication between the mixing tank and the holding tank. The valve is controlled by condition responsive means for operating the valve between open and closed positions in response to preselected conditions in the mixing tank.

United States Patent 1191 Marshall 1111 3,871,627 1451 Mar. 18, 1975 CHEMICAL MIXER [75] Inventor: Arthur R. Marshall, Orchard Lake,

Mich.

[73] Assignee: Chemix Corporation, Troy, Mich.

Primary Examiner-Harvey C. l-lornsby Attorney, Agent, or FirmMcGlynn and Milton [57] ABSTRACT Apparatus for mixing dry chemical with a liquid including a mixing tank having a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank. Wetting means is provided for wetting the dry chemical as it falls into the mixing tank, the wetting means including first spray means located on one side of the path of the dry chemical and second spray means located on the opposite side of the path of the: dry chemical. The streams of liquid from the first and second spray means intercept each other substantially in the path of the dry chemical so that the dry chemical passes through the intercepting streams and is wetted thereby as it falls into the mixing tank. The apparatus further includes a detention tank located beneath the mixing tank, and valve means for controlling communication between the mixing tank and the holding tank. The valve is controlled by condition responsive means for operating the valve between open and closed positions in response to preselected conditions in the mixing tank.

15 Claims, 7 Drawing Figures 1 CHEMICAL MIXER This invention relates generally to apparatus for mixing dry chemicals with liquids, and is particularly concerned with apparatus for producing accurately proportioned homogeneous solutions for use in various industrial processes.

A particular problem encountered in the mixing of dry granular chemicals with liquid is that the dry chemical tends to agglomerate and form into large lumps when it falls into the liquid with which it is to be mixed. The agglomeration prevents thorough mixing, and the solution is therefore not homogeneous. Furthermore, the agglomerates interfere with the mixing of subsequent batches of the material.

Polymers have been found to be a highly effective tool for use in the treatment of water and waste. It is, however, difficult to dissolve polymers in aqueous solutions. To assure complete dissolving of the polymer, it is necessary to prewet finely dispersed polymer particles prior to the time that the particles enter the aqueous bath or solution in order to reduce the tendency of the particles to agglomerate and form large globules or masses with only the outer periphery of each globule wetted. After prewetting the polymer particles as they fall into the liquid bath, it is then necessary to stir or agitate the bath to complete the dissolving ofthe polymer particles and obtain a homogeneous solution. Agitation or stirring of the solution must take place for a minimum period of time, after which the batch can be removed from the mixing tank to make way for the mixing of another batch. Andris U.S. Pat. No. 3,697,052 discloses apparatus concerned with the automatic control ofmixing of polymers and other difficult to dissolve chemicals into aqueous solution, the apparatus including a feeder for metering the dry chemical into the mixing tank at a controlled rate, prewetting means for prewetting the particles as they fall into the aqueous bath, agitating or stirring means for stirring the aqueous bath to complete dissolving ofthe dry chemical and the aqueous solution, liquid level sensing means for sensing the liquid level in the mixing tank, and automatic controls for controlling the mixing cycle in the mixing tank in accordance with the liquid level.

Examples of other apparatus of this general type, or ofapparatus having one or more ofthe latter type components, are disclosed in U.S. Pat. Nos. 2,872,941; 3,425,669; 1,787,289; 2,953,359; 3,433,464; 3,450,391.

In processes usingpolymers and similar dry granular chemicals, it is necessary to maintain a high rate of ac curately controlled automatic mixing of the dry chemicals with liquid to produce on demand homogeneous solutions for use in the processing.

In the prior U.S. Pat. No. 3,697,052 of Fred A. Andris, a mixing apparatus is disclosed which includes a mixing tank having prewetting means for wetting dry chemical as it is metered into the mixing tank, an agitator for mixing the prewetted chemical with the liquid in the mixing tank, level sensing means operable to activate mechanism for feeding the dry chemical into the mixing tank and to activate the prewetting means when the liquid is at a predetermined low level, and operable to deactivate the feed metering means and prewetting means when the liquid in the mixing tank is at a predetermined high level, and control means for controlling the period ofoperation of the agitator. When the agitatank is completed, and the batch is ready for discharge from the mixing tank.

One of the problems encountered with this type of apparatus is that the stream of liquid sprayed from the prewetting means creates air currents that cause dust from the granular chemical to collect on the surfaces of the apparatus which requires frequent cleaning in order to prevent interference with the operation of the components of the apparatus.

In other prior art apparatus of this type, the contents of the mixing tank are discharged from the mixing tank generally through a complex arrangement of external plumbing.

An object of this invention is to provide apparatus for mixing dry chemical with a liquid having a more efficient prewetting means for prewetting the dry chemical as it is metered into the mixing tank before it falls into the liquid bath contained in the mixing tank to reduce the amount of dust and to reduce the amount of agglomeration by the dry chemical.

A further object is to provide improved prewetting means for such apparatus that will not only reduce the amount ofdust, but will increase the capacity of the apparatus by increasing the rate of introduction of liquid into the apparatus.

A further object is to provide apparatus for mixing dry chemical with a liquid wherein the dry chemical is mixed with the liquid in a mixing tank and is then transferred to a holding or detention tank automatically upon completion of an automatically controlled mixing cycle.

In carrying out the foregoing, and other objects, apparatus according to the present: invention includes a mixing tank with a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank and falls downwardly into a bath of liquid contained in the mixing tank. Prewetting means is provided for prewetting the dry chemical as it falls into the mixing tank from the feed passage. The prewetting means includes first and second spray means located on opposite sides of the path of the dry chemical falling from the feed passage, the spray means each having streams which intercept each other in the path of the dry chemical so that the dry chemical passes through the intercepting streams and is wetted thereby as it falls into the bath contained in the mixing tank. The direction of flow of the streams of the spray means is such as to prevent the generation of air currents that would tend to create a dust problem within the apparatus, and the provision of the spray means is such as to increase the rate of flow of liquid into the mixing tank.

Further, in accordance with the invention, the apparatus includes a holding or detention tank located beneath the mixing tank which communicates with the mixing tank through an opening in the bottom wall of the mixing tank. Valve means is provided for controlling communication between the: mixing tank and the detention tank through said opening, and condition responsive control means is provided for operating the valve between open and closed positions in response to preselected conditions in the mixing tank.

Other objects, advantages and features of the invention will become apparent from the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a sectional elevational view of apparatus embodying the present invention;

FIG. 2 is a sectional view taken on lines 22 of FIG.

FIG. 3 is a view taken on lines 33 of FIG. 1;

FIG. 4 is an elevational view of the spray nozzle used in the prewetting assembly;

FIG. 5 is an elevational view, partially in section, of a second spray nozzle used in the prewetting assembly;

FIG. 6 is a schematic wiring diagram of a portion of the controls of the apparatus of FIG. 1; and

FIG. 7 is an elevational view of apparatus embodying the invention in another form.

The chemical mixing apparatus in FIG. 1 is desig nated generally by reference numeral 2 and includes an upper housing 4 for enclosing the mechanical components, a mixing tank 6 located beneath the upper housing 4, and a holding or detention tank 8 located beneath the mixing tank 6. Supported within the upper housing 4 is a hopper 10 for storing a quantity of the dry chemical in powdered form to be mixed with water or other liquid in the mixing tank 6. The upper end of the upper housing 4 is closed by a removable cover 12 which also overlies the upper end of the hopper 10. The upper housing 4 has a bottom wall 14 overlying the cover 15 of the mixing tank 6.

Enclosed within the upper housing 4 are various mechanical components, or portions thereof, of the apparatus, including a vibrator 16, a rotary feeder l8 driven by a motor through a chain 22, an agitator 24, valve means 26 for controlling communication between the mixing tank and holding tank, a level sensing device 27 and liquid supply controls 28. The rotary feeder l8 feeds the dry chemical from the hopper 10 through a feed passage 30 extending through the bottom wall 14 and cover 15 into the mixing tank 6 at a controlled rate in a thin curtain so that the individual particles can be prewetted by prewetting means to be described below before they fall into the liquid bath in the mixing tank 6.

The specific construction of the vibrator 16, rotary feeder l8, agitator 24, liquid level control device 27 and liquid supply controls 28 form no part of the present invention and may be of the same construction as the corresponding component shown in the previously referred to Andris U.S. Pat. No. 3,697,052, however, it being understood that the present invention is not limited to the specific construction and arrangement of these components.

The liquid level control device 27 includes a low liquid level probe 32, a ground probe 34, and a high liquid level probe 36. As in the previously referred to US. Pat. No. 3,697,052, when the liquid level is low enough to be out of contact with the low level probe 32, the mixing cycle is activated, and liquid is supplied through a pipe 38 from a supply (not shown) to the mixing tank through the prewetting assembly. After the liquid supply begins, the vibrator 16, feed metering device 18 and agitator 24 are started into operation. The feed meter- The prewetting means comprises first spray means 40 located on one side of the path of the dry chemical as it falls through the feed passage 30, and second spray means 42 located on the opposite side of the path of the dry chemical falling from the passage 30. The first spray means 40 is connected with the liquid supply controls through a conduit 44, and the second spray means 42 is connected with the liquid supply controls through a conduit 46. The first spray means 40 comprises a bank of parallel spray heads 48 depending vertically from a manifold block 50 supported beneath the cover 15. Each of the spray heads 48 is formed with a plurality of fluid outlets 52 (FIG. 5) through which the liquid is sprayed in a substantially horizontal direction toward the path of the dry chemical falling through the feed passage 30. The spray heads 48 are individually designated in FIG. 3 as spray heads 48a, b, c and d. The spray heads 48a, b, c and d are disposed angularly with respect to each other such that the stream of liquid from the spray head 4811, as indicated by the broken lines in FIG. 3, intersects the stream of liquid from the spray head 48c, and the streams of liquid from the spray heads 48b and d diverge outwardly from the block 50 toward the feed passage 30 and enclose the streams from the spray heads 48a and c. As illustrated in FIG. 1, the liquid is sprayed from the outlets 52 of the spray heads 48 in a substantially horizontal direction and falls toward the bottom of the mixing tank as it passes beneath the feed passage 30.

The second spray means 42 comprises a nozzle of conventional construction having a configuration to emit a fan-shaped stream downwardly toward .the path of the dry chemical falling from the feed passage 30, or downwardly and toward the right as shown in FIG. I. The nozzle 42 is formed with a constricted passage 52 and a deflector portion 54 (FIG. 4). As fluid is emitted from the constricted passage 52, it strikes the deflector portion 54 and is deflected downwardly and toward the right as viewed in FIGS. 1 and 4 and is formed into a fan-shaped spray by the deflector portion 54 as indicated by the broken lines 42a and 42b in FIG. 3. The fan-shaped stream from the nozzle 42 intercepts the streams from the spray heads 48 in the path of the dry chemical so that the curtain of dry chemical passes through the intercepting streams and is thoroughly prewetted as it falls into the bath contained in the mixing tank 6. Moreover, the intercepting streams create air currents tending to draw the dry chemical particles downwardly toward the intercepting streams thus reducing the tendency of the particles to spread outwardly from the feed passage and create a dust problem.

As the liquid level rises in the mixing tank 6, the solution is continuously stirred and agitated by the impeller 56 of the agitator 24. When the liquid level reaches the high level probe 36 of the level sensing device 27, the liquid supply is shut off and a timer in the control circuit is energized. When the timer stops, the agitator 24 stops and the valve means 26 is activated to cause the liquid in the mixing tank 6 to be discharged into the detention tank 8.

The valve means 26 is illustrated in detail in FIG. 2. An opening 60 is formed in the bottom wall 7 of the mixing tank, and the valve means 26 includes a tube 62 supported at its upper end adjacent to the bottom wall 14 of the upper housing 4 and extending through the mixing tank and opening 60 with its lower end projecting into the detention tank 8. The upper end ofthe tube 62 is secured by screws 64 to a mounting plate 66 overlying an opening 68 extending through the bottom wall 14 of the upper housing 4 and the cover member of the mixing tank 6. Vent holes 70 are formed in the tube 62 near the upper end thereof, and flow metering apertures 72 are formed in the tube 62 at a location just above the bottom wall 7 of the mixing tank 6.

The tube 62 and other components of the valve assembly are formed of organic plastic material such as polyethylene. Adhesively secured to the inner wall of the tube 62 is a cylindrical valve seat member 74. The upper edge 76 of the valve seat member 74 defines a valve seat and is chamfered downwardly and outwardly as indicated by reference numeral 78. The valve seat 76, as illustrated in FIG. 2, projects slightly above the lower edges of the flow metering apertures 72.

Flow from the mixing tank 6 through the flow metering apertures 72 and the valve seat 76 is controlled by a valve element 78 movable between an open position shown in FIG. 2 in which it is spaced from the valve seat 76, and a closed position in which the resilient facing 80 thereof is seated against the valve seat 76. The

valve element 78 is secured by conventional fasteners 81 to the lower, threaded end of a rod 82. The upper end of rod 82 is secured to the armature 84 of a solenoid 86. The armature 84 reciprocates within a cylindrical sleeve member 88 having a flange 90 secured by fasteners 92 to the plate 66. The rod 82 projects through a central opening in the plate 66 and is engaged by an annular sealing member 94 mounted in the central opening. Received in the sleeve member 88 is a spring 96 which has its lower end seated on the plate 66 and its upper end seated against the armature 34 to bias the valve element 78 to the open position shown in FIG. 2. When the solenoid 86 is energized, the armature 84 moves downwardly against the bias of spring 96 to move the valve element 78 to its closed position to shut off communication between the mixing tank 6 and the detention tank 8. The solenoid 86 is secured to the upper flange 91 of the sleeve member 88 by fasteners 93.

As pointed out above, when the solenoid 86 is energized, the armature 84 moves downwardly as viewed in FIG. 2 to actuate the valve element 78 to its closed position and shut off communication between the mixing tank 6 and the detention tank 8. When the current to the solenoid 86 is interrupted to deenergize the solenoid, the spring 96 extends and moves the valve element 78 to the open position shown in FIG. 2 to permit the liquid contents ofthe mixing tank 6 to flow through the apertures 72 and the valve seat member 74 into the detention tank 8.

FIG. 6 illustrates a wiring diagram of a portion ofthe circuitry for the automatic control of the apparatus 2. In FIG. 6, reference numerals 100 and 102, respectively, indicate power and ground lines controlled by main switch 101. Connected in the circuit of FIG. 6 is the solenoid 86 of the valve assembly 26, the motor of the agitator 24, the motor for the vibrator 16, the solenoid 98 of the liquid supply controls 28, a liquid level control relay 104, a timer and a DC converter 107 for the rotary feeder motor 20. Connected with the timer 105 is a master control relay 115.

The liquid supply solenoid 98, vibrator 16, converter 107 for driving the feed motor 20, and the timer 105 are connected in series with a pressure responsive 124b, the liquid supply solenoid 98, vibrator motor 16 and feed motor 20 through the DC converter 107 are energized to start a cycle of operation.

In the circuit diagram of FIG. 6, the contacts 114 are normally open and are closed when the master control relay 115 is energized through either the switch 118 or 119. When the contacts 114 are closed, the solenoid 86 and agitator motor 24 are energized. The contacts 108 of the liquid level control relay 104 and contacts 112 are normally closed at a low level condition in the mixing tanks 6,,and open when a high level condition is reached as sensed by the high liquid level probe 36. Contacts 110 of the level control relay 104 are normally open when a low level condition such as illus' trated in FIG. 1 is reached in the mixing tank 6, and closed when a high level condition is reached. The contacts 110 close when a high level condition is reached, and remain closed until the liquid level drops all the way to the low level condition as sensed by the low level sensing probe 36 to prevent recycling of the system as soon as the liquid level falls slightly below the high level condition. The contacts 110 thus act to hold the components in a condition for discharging the mixed liquid from the tank 6 into the holding tank 8 be-- tween the time that the level falls from a high level condition to the low level condition illustrated in FIG. 1. Contacts 106 and 112 are normally closed at low level. and these contacts-are opened when a high level condition is reached in the tank to interrupt the circuit to the liquid supply solenoid 98, vibrator 16, and feed motor 20.

,When the main switch 101 is closed, the master con- 'trol relay is energized through the switch 118 of the timer 105. Energization of the relay 115 causes the contacts 114 to close to energize the solenoid 86 and the agitator motor 24. As soon as the relay 115 is energized, the switch 118 opens and the switch 119 simultaneously closes to maintain a completed circuit to the relay 115 through switch 119. When the timer 105 completes its timing cycle, or times out, the switch 119 opens to deenergize the relay 115 which in turn opens the contacts 114 to interrupt the flow of current to the solenoid 86 and agitator motor 24.

If, during the mixing cycle, the: pressure in the liquid control system 28 drops to an inadequate level, the pressure switch 124 moves to the position shown in FIG. 6 to immediately stop the liquid supply, the vibrator motor 16, and the feed motor 20', however, the agitator motor 24 and the solenoid 86 remain energized so that the cycle will restart when the pressure is restored in the liquid supply system 28.

When the apparatus is to start into operation, the hopper 10 is filled with the dry polymer. With a low liquid level condition in the mixing tank 6 as illustrated in FIG. 1, and adequate pressure in the liquid supply system 28 to cause the switch 124 to move into contact with the contact 124b, Closing of the main switch 101 energizes the ready light 117, the timer 105, solenoid 86, agitator motor 24, indicator light 120, liquid supply solenoid 98, vibrator motor 16, and the converter 107 which in turn energizes the feeder motor to start a mixing cycle. Relay 115 is energized immediately upon the closing of the main switch 101 through the switch 118, after which the switches 119 and 121 of the timer 105 close and the switch 118 opens. The solenoid 86 and agitator motor 24 are energized upon energization of the relay 115 due to the closing of contacts 114. En ergization of the solenoid 86, as pointed out above, causes the valve element 78 to move to its closed position to shut off communication between the mixing tank 6 and detention tank 8. When the liquid level reaches the high level condition in the mixing tank 6, contacts 106, 109 and 112 open to shut off the flow of current to the indicator light 120, the liquid supply solenoid 98, the vibrator 16, and the feeder motor 20 through the converter 107. The timer then starts into a timing cycle, during which time, the relay 115, and

hence the solenoid 86 and agitator motor 24, remain energized. When the timing cycle has been completed, the switch 119 of the timer 105 opens to deenergize the relay 115 which in turn deenergizes the solenoid 86 to cause the valve element 78 to open and permit the contents of the mixing tank 6 to flow into the detention tank 8. When the low level condition is reached, the contacts 110 open, and the contacts 109 and 112 close, as does the switch 118 of the timer 105, to restart the mixing cycle.

The mixed solution in the detention tank 8 is withdrawn from the detention tank 8 through a pipe 99 by a metering pump on demand. Alternatively, the contents of the detention tank 8 can be transferred through the pipe 99 to a larter holding tank from which the mixed polymer can be distributed as needed in the particular process involved.

FIG. 7 illustrates a self-contained unit having a larger capacity detention tank with a plurality of metering pumps for distributing the mixed solution from the detention tank as needed. The apparatus of FIG. 7 is designated generally by reference numeral 132 and includes an upper housing 134 and mixing tank 1360f substantially identical construction and having the same components as the embodiment of FIG. 1. The apparatus is secured to a detention tank 138 for receiving the contents of the mixing tank 136 after each mixing cycle, and mounted on the holding tank 138 is a plurality of metering pumps 139, 131 and 133. The metering pumps selectively withdraw the mixed solution from the holding tank 138 for distribution through selected lines connected with each pump.

The invention is, of course, not limited to the specific sequence of operation described above in connection particularly with FIG. 6. For example, the timer 105 can be operated to begin a timing cycle as soon as filling of the mixing tank 6 begins so long as the agitator 24 operates for a specific time after the mixing tank is filled.

Furthermore, it has been found that the cylindrical sleeve member 88 (FIG. 2) is in some cases subjected to unduly high stresses particularly during shipment and installation. Accordingly, to alleviate this condition, the sleeve 88 can be recessed into the sleeve 62 with the flange 91 supported directly on plate 66.

While specific forms ofthe invention have been-illus trated and described in the foregoing specification and' accompanying drawings, it should be understood that the invention is not limited to the exact construction shown, but that various alterations and modifications in the construction and arrangement of parts, all falling within the scope and spirit of the invention, will be apparent to those skilled in the art.

I claim:

1. Apparatus for mixing dry chemical with a liquid comprising: a mixing tank having a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank and falls toward the bottom of the mixing tank into any liquid contained therein; and prewetting means for prewetting the dry chemical as it falls into the mixing tank from said passage, said prewetting means comprising a plurality of spray heads located on one side of the path of dry chemical falling from said passage and including a plurality of fluid nozzles for spraying liquid generally horizontally into the path of the dry chemical, and a fan nozzle located on the opposite side of the path of dry chemical falling from said passage for spraying liquid in a fan-shaped stream generally downwardly toward said bottom.

2. Apparatus as claimed in claim 1 wherein said plurality of parallel spray heads depend toward the bottom of said mixing tank, each of said spray heads being formed with a plurality of said fluid nozzles defined by outlets through which liquid is sprayed substantially horizontally toward the path of dry chemical.

3. Apparatus as claimed in claim 1 further including a detention tank located beneath said mixing tank, an opening in the bottom wall of said mixing tank, valve means for controlling communication between said mixing tank and said detention tank through said opening, and condition responsive means for operating said valve between open and closed positions in response to preselected conditions in said mixing tank.

4. Apparatus as claimed in claim 3 wherein said valve means comprises a tube in said mixing tank extending through said opening, a valve seat in said tube near said opening, flow metering apertures in the wall of said tube above said valve seat providing communication between said mixing tank and the interior of said tube, and a valve element in said tube movable between open and closed positions with respect to said valve seat to respectively permit flow from said mixing tank through said apertures and said valve seat into said detention tank, and to shut off communication between said mixing tank and detention tank.

5. Apparatus as claimed in claim 4 wherein said valve means further includes a spring biasing said valve element to its open position, and said condition responsive means includes a solenoid operable when energized to actuate said valve to its closed position against the bias of said spring.

6. Apparatus as claimed in claim 5 wherein said valve seat comprises a cylindrical member secured to the inner wall of said tube near said opening, the upper edge thereof being chamfered downwardly and outwardly to define said valve seat.

7. Apparatus as claimed in claim 5 further including level sensing means in said mixing tank, said solenoid being electrically connected with said level sensing means such that said solenoid is energized to close said valve element at a preselected low liquid level in said mixing tank and is deenergized to permit said valve element to open at a preselected high liquid level in said mixing tank.

8. Apparatus for mixing dry chemical with a liquid comrpising: a mixing tank having a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank and falls toward the bottom of the mixing tank and any liquid contained therein; metering means for metering the dry chemical into the mixing tank such that the dry chemical enters the mixing tank at a controlled rate; prewetting means for wetting the stream of dry chemical as it is fed into the mixing tank through said feed passage by said metering means; agitating means in said mixing tank for agitating and mixing the liquid and dry chemical in the mixing tank; a detention tank located beneath said mixing tank; an opening in the bottom wall of said mixing tank; valve means controlling communication between said mixing tank and said detention tank through said opening; and condition responsive control means for operating said valve means between open and closed positions in response to preselected conditions in said mixing tank.

9. Apparatus as claimed in claim 8 wherein said valve means comprises a tube in said mixing tank and extending through said opening, a valve seat in said tube near said opening, flow metering apertures in the wall of said tube above said valve seat providing communication between said mixing tank and the interior of said tube, and a valve element in said tube movable between open and closed positions with respect to said valve seat to respectively permit flow from said mixing tank through said apertures and said opening into said detention tank, and to shut off communication between said mixing tank and detention tank.

10. Apparatus as claimed in claim 9 wherein said valve seat comprises a cylindrical member secured to the inner wall of said tube near said opening, the upper edge thereof being chamfered downwardly and outwardly to define said valve seat.

11. Apparatus as claimed in claim 10 wherein said valve means further includes a spring biasing said valve element to its open position.

12. Apparatus as claimed in claim 11 wherein said condition responsive control means includes a solenoid operable when energized to actuate said valve element to its closed position against the bias of said spring.

13. Apparatus as claimed in claim 12 wherein said condition responsive control means further includes level sensing means electrically connected with said solenoid such that said solenoid is energized to close said valve element at a preselected low liquid level in said mixing tank and is deenergized to permit said valve element to open at a preselected high liquid level in said mixing tank.

14. Apparatus as claimed in claim 13 wherein said condition responsive control means further includes a timer electrically connected with said agitator and said solenoid operable to interrupt operation of said agita tor and deenergize said solenoid after a preselected period of operation of said agitator.

l5. An-apparatus for mixing dry chemical with a liquid wherein the dry chemical is metered into a mixing tank through a feed passage at a controlled rate and is prewetted by a prewetting assembly as it falls toward the bottom of the mixing tank, the improvement wherein said prewetting assembly comprises a plurality ofspray heads located on one side of the path of the dry chemical falling from said passage and including a plu rality of fluid nozzles for spraying liquid into the path of the dry chemical in a substantially horizontal direction and a fan nozzle located on the opposite side of the path of the dry chemical for spraying liquid in a fanshaped stream generally downwardly. 

1. Apparatus for mixing dry chemical with a liquid comprising: a mixing tank having a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank and falls toward the bottom of the mixing tank into any liquid contained therein; and prewetting means for prewetting the dry chemical as it falls into the mixing tank from said passage, said prewetting means comprising a plurality of spray heads located on one side of the path of dry chemical falling from said passage and including a plurality of fluid nozzles for spraying liquid generally horizontally into the path of the dry chemical, and a fan nozzle located on the opposite side of the path of dry chemical falling from said passage for spraying liquid in a fanshaped stream generally downwardly toward said bottom.
 2. Apparatus as claimed in claim 1 wherein said plurality of parallel spray heads depend toward the bottom of said mixing tank, each of said spray heads being formed with a plurality of said fluid nozzles defined by outlets through which liquid is sprayed substantially horizontally toward the path of dry chemical.
 3. Apparatus as claimed in claim 1 further including a detention tank located beneath said mixing tank, an opening in the bottom wall of said mixing tank, valve means for controlling communication between said mixing tank and said detention tank through said opening, and condition responsive means for operating said valve between open and closed positions in response to preselected conditions in said mixing tank.
 4. Apparatus as claimed in claim 3 wherein said valve means comprises a tube in said mixing tank extending through said opening, a valve seat in said tube near said openIng, flow metering apertures in the wall of said tube above said valve seat providing communication between said mixing tank and the interior of said tube, and a valve element in said tube movable between open and closed positions with respect to said valve seat to respectively permit flow from said mixing tank through said apertures and said valve seat into said detention tank, and to shut off communication between said mixing tank and detention tank.
 5. Apparatus as claimed in claim 4 wherein said valve means further includes a spring biasing said valve element to its open position, and said condition responsive means includes a solenoid operable when energized to actuate said valve to its closed position against the bias of said spring.
 6. Apparatus as claimed in claim 5 wherein said valve seat comprises a cylindrical member secured to the inner wall of said tube near said opening, the upper edge thereof being chamfered downwardly and outwardly to define said valve seat.
 7. Apparatus as claimed in claim 5 further including level sensing means in said mixing tank, said solenoid being electrically connected with said level sensing means such that said solenoid is energized to close said valve element at a preselected low liquid level in said mixing tank and is deenergized to permit said valve element to open at a preselected high liquid level in said mixing tank.
 8. Apparatus for mixing dry chemical with a liquid comrpising: a mixing tank having a feed passage defined above the bottom wall thereof through which dry chemical is fed into the mixing tank and falls toward the bottom of the mixing tank and any liquid contained therein; metering means for metering the dry chemical into the mixing tank such that the dry chemical enters the mixing tank at a controlled rate; prewetting means for wetting the stream of dry chemical as it is fed into the mixing tank through said feed passage by said metering means; agitating means in said mixing tank for agitating and mixing the liquid and dry chemical in the mixing tank; a detention tank located beneath said mixing tank; an opening in the bottom wall of said mixing tank; valve means controlling communication between said mixing tank and said detention tank through said opening; and condition responsive control means for operating said valve means between open and closed positions in response to preselected conditions in said mixing tank.
 9. Apparatus as claimed in claim 8 wherein said valve means comprises a tube in said mixing tank and extending through said opening, a valve seat in said tube near said opening, flow metering apertures in the wall of said tube above said valve seat providing communication between said mixing tank and the interior of said tube, and a valve element in said tube movable between open and closed positions with respect to said valve seat to respectively permit flow from said mixing tank through said apertures and said opening into said detention tank, and to shut off communication between said mixing tank and detention tank.
 10. Apparatus as claimed in claim 9 wherein said valve seat comprises a cylindrical member secured to the inner wall of said tube near said opening, the upper edge thereof being chamfered downwardly and outwardly to define said valve seat.
 11. Apparatus as claimed in claim 10 wherein said valve means further includes a spring biasing said valve element to its open position.
 12. Apparatus as claimed in claim 11 wherein said condition responsive control means includes a solenoid operable when energized to actuate said valve element to its closed position against the bias of said spring.
 13. Apparatus as claimed in claim 12 wherein said condition responsive control means further includes level sensing means electrically connected with said solenoid such that said solenoid is energized to close said valve element at a preselected low liquid level in said mixing tank and is deenergized to permit said valve element to open at a preselected high liquid level in said mixing Tank.
 14. Apparatus as claimed in claim 13 wherein said condition responsive control means further includes a timer electrically connected with said agitator and said solenoid operable to interrupt operation of said agitator and deenergize said solenoid after a preselected period of operation of said agitator.
 15. An apparatus for mixing dry chemical with a liquid wherein the dry chemical is metered into a mixing tank through a feed passage at a controlled rate and is prewetted by a prewetting assembly as it falls toward the bottom of the mixing tank, the improvement wherein said prewetting assembly comprises a plurality of spray heads located on one side of the path of the dry chemical falling from said passage and including a plurality of fluid nozzles for spraying liquid into the path of the dry chemical in a substantially horizontal direction and a fan nozzle located on the opposite side of the path of the dry chemical for spraying liquid in a fan-shaped stream generally downwardly. 